This invention provides a directional-antenna-placement visual aid and method of placing directional antenna.
Radio frequency (RF) electromagnetic signals are used for real-time two-way communication. A signal in a chosen frequency band can be modulated to produce an analog or a digital signal. Portions of a signal can be spread across more than one frequency band. Signals can be combined and transmitted on the same frequency band or bands essentially simultaneously, and each of a large number of various receivers can isolate the signal of interest from the combined signal. The ability to combine and separate several individual signals on the same frequency band is the basis for cellular telephone service and for similar communications methods such as private or governmental communications systems.
In contrast to powerful single-point radio-frequency (RF) transmitters and receivers, cellular telephone service employs a large number of widely distributed base-station antennae serving relatively small cells at relatively low power. These are commonly seen in the form of cellular telephone towers or can be mounted on, or in, buildings. In theory, any given frequency band may be safely re-used for any antenna at a sufficient distance from, or otherwise isolated from, another antenna using that frequency band.
Although omnidirectional antennae can be used for cellular telephone base stations, the common practice is to use directional antennae having defined angles of coverage in order to better allocate and control the spatial distribution of the RF signals.
The cellular telephone networks possess a relatively small number of radio-frequency bands, or channels, to serve a relatively immense number of users of mobile cellular telephones. Each cellular telephone performs background communications with the cellular network, so that the network knows that a given cellular telephone is in communication with a specific base-station or tower, and the cellular network does not waste bandwidth or channels sending that user's signals through base stations that are physically remote from that cellular telephone's reported location. In order to accommodate more channels, and therefore more users, in physical areas densely populated with users, the normal-sized cell is subdivided into smaller cells, such as picocells and femtocells. The subdivisions allow frequency bands or channels to be re-used within a closer proximity to each other.
If the number of active cellular telephones in a given area exceeds the number of available channels of bandwidth in that area, many of the cellular telephones will experience difficulty with placing, receiving, and continuing calls without dropping. For this reason, many large venues for large gatherings of people, such as business meetings and expositions, sports, and entertainment, are equipped with a large number of antennae creating multiple subdivided cells within the venue.
Cellular telephones can detect signals from more than one base station or tower, especially within smaller, subdivided cells. Generally, one base station will have a significantly stronger signal, and the cellular telephone will be able to recognize and filter out the competing weaker signals, isolating the stronger signal. But in some circumstances a cellular telephone will not be able to isolate just one signal, and will present more than one signal to the user's cellular telephone, as cross talk, noise, or dropped calls.
The strength of radio frequencies received by any given cellular telephone in any given place varies greatly because of a number of factors, from large, such as whether the cellular telephone is on a hilltop or on one side or another of the hill, to small, such as how much of a user's hand covers the antenna portion of the handset. When a cellular telephone is receiving similar-strength signals from more than one base station or tower, the relative strengths of those signals is likely to vary, with the positions of weaker and stronger signals changing back and forth.
In order to accommodate the movement of cellular telephones from one base station or tower, to another, even during the progress of a telephone call, a method of soft handover or handoff is employed by the cellular networks. In normal circumstances such as driving down the highway, the cellular telephone will be mapped to a specific base station at any given point in time, and will eventually leave that base station behind. As the signal from the next, forward tower becomes stronger, the cellular network will reserve a channel on that forward tower for the approaching cellular telephone. At some point between the two towers, the next tower will become the primary tower. But the behind tower does not release the channel immediately, because of the possibilities that the apparent strength of the signal from the forward tower was only a temporary condition, and the transfer to the forward tower fails, in which case the behind tower will continue to be or resume being the primary tower for service to that user.
The soft handover method, where an additional channel on a weaker-signal tower is reserved for a user in anticipation of the user moving toward that tower, and where the channel is maintained on the formerly stronger-signal tower even after it becomes the weaker-signal tower, means that one cellular telephone will tie up more than one channel of the cellular network's bandwidth during the time the cellular telephone remains in a position between two or more towers. In circumstances where, for example, a user is approaching an area between two or more different forward towers, it is possible that the user will tie up even more than two channels of bandwidth during the transition.
Under the circumstances of a cellular telephone traveling in a vehicle, the periods of transition are relatively brief, and the additional bandwidth burden on the cellular network is seen as a reasonable trade-off for the avoidance of dropped calls.
In a large venue equipped for cellular telephone coverage, however, the cellular telephones are much less mobile, and may not move at all, for a user sitting watching a sports or entertainment event, or may move at a walking pace with frequent stops for a business exhibition. If any given cellular telephone comes to rest in, or travels slowly through, an area covered by signals from more than one antenna from the relevant cellular network, then that one cellular telephone will linger in the soft-handover condition, tying up two or more channels of bandwidth, for a long time. When the user shifts position or moves the cellular telephone, the shifting of the relative signal strengths may trigger a fresh soft handover.
For the cellular networks, the large-venue issue of a large number of cellular telephones each tying up two or more channels of bandwidth for very long periods is an issue of expense and of quality of service. The number of available channels is finite and relatively small. The soft-handover protocol is already established and is programmed into all handsets and switching equipment. Further subdivision of cells into even smaller cells is limited by available bandwidth and by the undesirable consequence that even more cells would mean even more overlapping signals and more soft handovers.
The installation of large numbers of cellular-telephone RF antennae in large venues, with the intent of providing as much coverage as possible, can have the unfortunate effect of creating areas of overlapping signals of nearly equivalent strength, putting a great many stationary cellular telephones into a soft-handover condition, with each cellular telephone tying up at least two channels for the entirety of the time. The effects of overlapping signals are not just a misallocation of a cellular network's bandwidth. The overlapping signals lead to a degradation of the quality of service affecting cellular telephone users in several ways. Besides the tying up of channels, which leads to dropped calls and low quality of service, the extraneous RF signals can have phase-cancelling effects on the desired signal at the cellular telephone, and can produce crosstalk and noise at the cellular telephone.
Although the individual antenna elements for cellular telephone RF are only inches long, the standard directional antenna array is approximately one meter or three feet in the longest dimension, nominally the vertical dimension, one-third of that in width, and one-sixth of that in depth. The sending-and-receiving or coverage angle of a standard cellular telephone RF directional antenna is likely to be in the range of 60 to 70 degrees in the longest, nominally vertical dimension, and 20 to 30 degrees in the perpendicular dimension. Antenna arrays generally have flat backs, which reduce their profile, and receptacles for power and signal connections and for a variety of mounts for physical mounting to towers and to outside and inside walls of buildings.
Directional antennae have known angles of coverage and are capable of being aimed. Although they are usually seen mounted with the longest dimension and coverage vertical, they can also be mounted with the longest dimension and coverage horizontal.
But RF signals are invisible, making precise aiming difficult. The present state of the art in large venues is to mount the antennae, then to have a technician walk every row and aisle of the venue with a test device that records the signal strengths at each individual position, and then to upload all of that data into a computer to produce an analysis that can identify areas of too little or too much coverage. From that analysis, antennae can be re-mounted or re-positioned for another try, and then the process of walking the venue and analyzing the data must be performed again.